The present invention is directed to soy protein based adhesives useful in the manufacture of particleboard and other composites. More particularly, the present invention is directed to a water resistant soy protein based adhesive containing a vegetable oil derivative.
Soy protein was used as an adhesive ingredient in plywood in the early 1900s. However, the problem of low moisture resistance led to its replacement with petroleum-based resins in the 1930s. Particleboard composites were subsequently developed that utilized urea-formaldehyde resins to bind the wood particles rather than agricultural adhesives.
However, increasing environmental awareness and the recognized hazards of formaldehyde-based adhesives has created a strong demand for more environmentally friendly wood adhesives. Successful replacement of urea formaldehyde resins in particleboard requires an adhesive that can produce composites having characteristics matching or exceeding those attainable with urea formaldehyde. Accordingly, water resistance is a necessary characteristic of any suitable replacement.
Soybean proteins are a combination of 18 amino acids of wide chemical diversity such as amines, carboxylic acids, hydroxyls and mercaptan functionalities. Each of these functionalities is polar and contributes to water sensitivity. Accordingly, in order to use soy protein as an effective adhesive, it is necessary to counteract this sensitivity. Additionally, the soybean binder must be chemically transformed into a hydrophobic adhesive without creating toxic by-products and/or waste streams.
In view of the foregoing, it would be a significant advancement in the art to provide an agriculturally based adhesive which could be utilized to replace urea formaldehyde resins in particleboard and other composites. It would be a further advancement in the art if such an adhesive was primarily based upon renewable resources. Such an adhesive is disclosed and claimed herein.
The present invention is directed to a soy protein based adhesive useful in the formation of particleboard and other composites. The adhesive contains a vegetable oil derivative to impart water resistance. In a preferred embodiment, the adhesive comprises a mixture of soy protein isolate, a polyol plasticizer, and a vegetable oil derivative. The preferred vegetable oil derivatives are maleinized vegetable oils.
The present invention is directed to a novel binder suitable for use in making particleboard and other composite materials. In a preferred embodiment, the binder of the present invention comprises a mixture of soy protein such as soy protein isolate, a plasticizer, and a vegetable oil derivative. The plasticizer is preferably a polyol such as glycerol. The vegetable oil derivative preferably is a fatty acid or oil which has been modified to include an anhydride pendent group.
Soy proteins must be combined with other ingredients in order to form suitable adhesive compositions. Soybean proteins are inherently unable to flow at elevated temperatures and pressures because of inter- and intra-molecular hydrogen bonds. Accordingly, the hydrogen bonding of the proteins must be disrupted with plasticizers for improved molecular mobility and a reduction in the pressure that is required to initiate flow. Polyhydric alcohols are known to be excellent plasticizers for soy protein.
Many different types of polyols can be used in the present invention. Suitable examples are 1,6-hexanediol, 2,5-hexanediol, 2-methyl-2-hexanol, 1,4-butanediol, 2,3-butanediol, 1,3-propanediol, 1,2-propanediol, 1,2,3-propanetriol (glycerol), 2-ethyl-2-(hydroxymethyl)-1,3-propanediol (trimethylol propane), and 1,2-ethanediol. Glycerol is a particularly preferred plasticizer for the present invention since it is non-toxic and is a renewable resource readily obtainable from vegetable oils.
When low concentrations of glycerol, i.e., 10-30 weight percent, are blended with soy protein isolate, they produce a homogeneous powder with only slightly higher particle cohesion than soy protein isolate alone. The particles are easily separated under low shear conditions which is an important factor for an adhesive to be blended with wood furnish to form particleboard.
When plasticizers are added to soy protein isolate, they improve flow but reduce Tg. Accordingly, it is desirable to add compounds which can counteract the polyol effects and generate an adhesive less flexible after it has been cured. It is important for a particleboard binder to possess some flexibility. However, excessive flexibility produces creeping under load. Many different compounds can be used to increase adhesive stiffness such as lignin and its derivatives, and conventional fillers such as ground walnut shells, calcium carbonate, and silicates. Of these, lignin is preferred since it is the polymeric material responsible for the stiffness of trees and vegetation, and is non-toxic and a renewable resource. A particularly preferred form of lignin for use in the present invention are lignosulfonates.
In order to use soy protein as a suitable adhesive in applications such as particleboard, it is necessary to improve the water resistance of the adhesive composition. Vegetable oil derivatives, when suitably modified, make ideal precursors from which to synthesize water-resistant additives. Vegetable oils are ideal precursors because they are nontoxic, biodegradable, renewable, nonvolatile at temperatures required to cure with composites, and are able to undergo chemical modification via double bonds located on the fatty acid residues.
Vegetable oils are comprised of one glycerol molecule bonded to three fatty acids via ester linkages. The most common fatty acids in vegetable oils are oleic, linoleic, linolenic and eleostearic. It has been found that by chemically altering the oils to introduce sites that react with protein nucleophiles and polyol plasticizers, water resistant adhesive formulations can be formed.
Many different compositions can be used to form vegetable oil derivatives. These include organic acids, acylhalides, anhydrides, aldehydes, ketones and epoxides. The preferred components for modifying the vegetable oils are the anhydrides. Cyclic, dehydrated di-acids were found to be advantageous for several reasons. First, they possess increased reactivity over organic acids and epoxides. Second, the ring-opening reaction does not produce unwanted by-products. Third, they produce bonds which are more stable than those formed by aldehydes and ketones. Finally, anhydrides can be attached to vegetable oils via Diels-Alder reactions through a binary reaction without solvents and waste streams.
In a preferred embodiment, vegetable oils containing eleostearic acid are utilized since the conjugated double bonds easily react with maleic anhydride. Tung oil is a preferred vegetable oil for use in the present invention because of the high concentration of eleostearic acid. The methyl ester of tung oil is even more preferred since it has a lower viscosity than tung oil after being maleinized. Additionally, a modified linseed oil known as Archer 1 available from Archer Daniels is also particularly suitable for use in the present invention.
In a preferred embodiment, the vegetable oil derivative is synthesized by heating the methyl ester of tung oil to 80xc2x0 C., adding excess maleic anhydride while stirring and allowing the components to react for 6-8 hours, removing unreacted maleic anhydride via sublimation, and determining anhydride equivalent weights by assaying the vegetable oil derivatives with standardized methanolic alkali. Such a procedure produces a maleinized methyl ester of tung oil having an anhydride equivalent weight of about 410 g/mol. A similar procedure can be used to maleinize Archer 1 and produce a product having an anhydride equivalent weight of about 890 g/mol.
Any vegetable oil derivative that contains anhydride functionality will work in the present invention.
The adhesive compositions of the present invention can be formulated by existing techniques. For example, a ceramic ball mill can be used to mix the ingredients. Additionally, a mortar and pestle can be used to mix small amounts. In a preferred embodiment the binder composition of the present invention comprises 50-80% by weight soy protein, 20-30% by weight plasticizer and 10-20% by weight vegetable oil derivative. The composition can also include lignin. In a particularly preferred embodiment, the binder composition comprises about 60% by weight soy protein, about 20% by weight plasticizer, about 15% by weight vegetable oil derivative and about 5% by weight lignin.
The invention is illustrated by the following examples which describe preferred embodiments of the present invention. However, it should be appreciated that the examples are illustrative only and are not limiting as to the scope of the invention.